Amplifier



April 25, 1933.

H. HOLLERITH, JR

AMPLIFIER 3 Sheets-Sheet 1 Filed Dec. 17 1927 April 25, 1933.

H. HOLLERITH, J R

AMPLIFIER Filed Dec. 17 1927 3 Sheets-Sheet 2 AMPLIFIER Filed Dec. 17, 1927 3 Sheets-Sheet 3 Patented Apr. 25, 1933 UNITED STATES PATENT OFFICE HERMAN HOLLERITH, J R., OlE RIVERTON, NEW JERSEY, ASSIGNOR, BY MESNE ASSIGN- MENTS, TO RADIO CORPORATION OF AMERICA, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE AMPLIFIER Application filed December 17, 1927. Serial No. 240,862.

for example,'where the useful frequencies range from a few cycles to several thousand cycles per second, transformer distortion is in some cases very marked, and the characteristic of transformer-coupled amplifiers falls off very rapidly above and below an intermediate range defined by the design of the amplifier unit and the capabilities of the transformers.

The directly coupled circuit, constituting a 33 direct-potential amplifier, eliminates all transformers and the consequent distortion, and has a flat characteristic over a much wider range than the inductively-coupled circuit. Such amplifiers are therefore very desirable for many uses such as radio amplifiers, amplifiers for electric sound reproducing machines, etc. Heretofore, however, it has been necessary to employ either a blocking condenser or a C or bias battery, in

' order to prevent the direct potential applied to the plate of one tube from getting on the grid of the succeeding tube because of the direct connection between said plate and grid. V'Vith the condenser a grid leak resistance must be used in order to prevent the tube from blocking; nevertheless such units, unless very carefully built, cause serious trouble due to blocking of the tubes from radio frequencies from the radio set or set up with- 40 in theainplifier itself. As a result, and in spite of its advantages, the directly coupled amplifier unit has never been extensively used in practice.

Rectified alternating current asa power supply for vacuum tubes may be used, where desired, to operate a complete amplifying unit from a single source of power. This may be conveniently done, for example, by connecting a potentiometer arrangement across said source and by tapping the respective grids, plates, and filaments on said resistance at points of the proper potential. Such a power supply is particularly well adapted for use with directly coupled ampli fiers, making possible an amplifying unit of very great efficiency with desirable simplicity of individual units and wiring, and consequent reliability and economy of construction, ifthe directly coupled amplifier is made capable of satisfactory operation.

Furthermore, it has been found that below a certain limit, the best designed and constructed amplifying systems operate with rapidly decreasing efliciencies. Organ tubes, for example, have frequencies sometimes as low as 16 cycles per second. When music having such extremely low frequencies is reproduced, it is found that the intermediate and higher frequencies are relatively overemphasized and the low frequencies relatively suppressed, producing a distorted effect. This distortion, while due in some measure to the characteristics of the amplifiers proper, is also due to a large extent to other factors, particularly loud speaker or horn characteristics, which add to such suppression until the reproduction is unsatisfactory. It is therefore highly desirable, even with the directly coupled amplifier, to reinforce the low frequencies.

Among the objects of the present invention is to provide a novel and improved directly coupled amplifier operating efiiciently and without distortion over a wide range of frequencies and capable of general application for amplification purposes.

Another object is to provide a directly coupled amplifier in which blocking of the tubes is effectively prevented by elimination of all locking condensers, bias batteries, etc.

Another object is to provide a directly coupled amplifier in which the blocking of the tubes is effectively prevented by the elimination of all coupling condensers without introducing any large and detrimental capacity to ground such as that of the grid batteries heretofore necessary in place of a blocking condenser.

Another object is to provide a novel combination of power supply means with a directly coupled amplifier whereby a multistage amplifier is operated from a single power source, the whole being of efficient and simple design and requiring a minimum of equipment, wiring connections, etc.

A further object is the provision in an amplifier of novel means for relativelygreat amplification of currents of particular frequencies such as very low frequency currents.

Other objects will appear more fully hereinafter as the description of the invention proceeds.

With the above objects in view, the terminals of a suitable unit source of power supply, such as rectified alternating current or direct current, are connected across any suitable form of potentiometer arrangement to constitute a source of graduated D. C. potential which supplies the amplifier proper. The amplifier, preferably a multistage amplifier, is of the directly coupled type, having the plate of each tube direct-connected to the grid of the succeeding tube, each plate-grid circuit preferably having a single voltage tap to a suitable point on said potentiometer arrangement. The tube filaments may likewise be shunted across portions of the potentiometer arrangement, or may be supplied from other sources such as raw alternating current, with connections to said potentiometer arrangement to complete the plate and grid circuits. In any case, the proper bias on the grid of each tube may be obtained by properly positioning either the filament connection or the plate-grid connection along the potentiometer arrangement. For the purpose f reinforcing particular frequencies, a selective amplifying circuit with or without feedback may be provided, preferably comprising an additional tube operated from the potentiometer arrangement and responsive at the desired frequencies, which is coupled to input of one stage of the amplifier.

Several embodiments of the invention have been illustrated in the accompanying drawings, but it is to be expressly understood that said drawings are for purposes of illustration only and are not to be taken as a definition of the limits of the invention, reference being had to the appended claims for that purpose.

In the drawings- Fig. 1 shows diagrammatically a multistage amplifier embodying the invention;

Fig. 2 shows diagrammatically a multistage amplifier similar to that shown in Fig. 1, and including means for reinforcing particular frequencies; and

Fig. 3 shows the application of the invention to a multistage amplifier of the push-pull type. i

As hereinbefore indicated, any suitable source. of either direct or alternating current may be employed. Preferably, and in the form shown, the amplifier is adapted to be operated from a suitable source of alternating current such as is commonly used in house lighting current. and for this purpose is provided with any suitable means for rectifying such current. Referring to Fig. 1, the amplifier may be connected to a suitable source of current by means of an ordinary connecting plug 1, from which current is conducted by parallel circuits 2, 2' to the primaries 3, 3 of suitable transformers, each of said circuits preferably including a ballast tube 4, 4. The secondary winding 5, 5 are each connected to the anodes 6. 7 and 6, 7 of suitable rectifying tubes. The filaments 8, 9 and 8. 9 of said tubes are preferably heated by means of transformer secondaries 10, 10. Rectifying circuits of this type are well known in the art and require no further explanation. The positive leads 11, 11 and the negative leads 12. 12 are preferably bridged by parallel condensers 13. 13 and 14, 1 1 and choke coils 15, 15 are interposed in each line between said. condensers. the parallel condensers and choke coil constituting filter circuits of a. well known type. It will be obvious, however, that the power circuit need not be divided and that all the stages may be operated from a single transformer, as shown in Fig. 3.

Referring now particularly to Fig. 1, there is shown by way of example. a three-stage directly coupled amplifier unit wherein the operating potentials for the tubes are supplied entirely from a suitable potentiometer arrangement connected directly across the terminals 11. 12 and 11, 12. Obviously said potentiometer arrangement may comprise a single unit. or. as shown, a plurality of resistance units of suitable value connected in series with the positive and negative leads. Said potentiometer arrangement serves as a power supply for three amplifying stages in cascade. comprising the amplifying tubes 16. 17 and 18. Each of the tubes of said series preferably has its plate connected directly to the grid of the succeeding tube. without the interposition of blocking condensers. bias batteries. etc. The plate to grid circuits lie-- tween each two tubes and the filament of each tube are connected directly to suitable points on the potentiometer arrangement whereby the proper plate potentials and the proper bias between the grid and filament may be obtained.

Referring now in greater detail to the construction of the amplifier proper, the input terminals 19. 20 are connected respectively to the grid 21 of tube 16 and to the negative side of the potentiometer arrangement. The filament 22 is shunted across a portion 23 of the resistance such as to provide the proper operating current and suitable potential therefor. It will be observed that the grid 21 is negative with respect to the filament 22' be .end of the potentiometer arrangement.

cause of the potential drop in the resistance 24. The plate 25 is connected, preferably through resistance 26, to a suitable point 27 on the potentiometer arrangement such that a suitable potential positive with respect to the filament 22 is impressed on the plate 25.

Because of the direct connection between the plate 25 and the grid 28 of the succeeding tube 17 the plate potential of the tube 16 is impressed directly on the grid 28. In order to provide suitable relative potential between the grid 28 and the filament 29, suitable means are provided for adjusting the potential of the filament 29 to a value which is preferably positive with respect to the grid 28; and, as in the case of the filament 22 of the preceding tube, the filament 29 may be shunted across a portion of the potentiometer arrangement which will provide the desired potential. The point of connection of the filament 29 with the potentiometer arrangement will ordinarily be somewhat negative with respect to the point 27, to allow for the drop in grid potential in resistance 26. Preferably, and as shown in Fig. 1, filament 29 may be heated from a transformer secondary 30. and in such case the filament heating circuit, being electrically insulated. may be suitably connected as by means of a lead 31 to a point of desired potential on the potentiometer arrangement. Preferably the lead 31 is con.- nected to the center point of a potentiometer 32 which is shunted across the filament heat ing circuit, in order to prevent any interference from humming due to the altern atine current employed. In the form shown, wherein the divided power circuit is employed, the plate 33 of tube 1.. may be connected through a resistance 34 to the positive It will be obvious that any potentialsof grid 28, filament 29 and plate may be obtained by properly positioning their relative points of connection to the potentiometer arrangement.

As herein shown, the plate 35 of the tube 18 is connected through the primary 36 of the output transformer to the positive lead 11, thereby receiving the full operating potentialof one side of the power circuit. The

filament 36 is heated by alternating current 7 from a secondary 37, and is connected by means of a potentiometer 38 with the negative lead 12'. In order to provide the proper potential relation between the filament 36 and the grid 39, the potentiometer 38 is connected by means of a lead 40 with a point of suitable potential on the potentiometer arrangement. The resistance 41 may be connected across the leads 11, 12 to stabilize the circuit.

Wherever it is desirable to prevent any interference from alternating current, suitable by-pass condensers such as the condensers 42 may be provided. Obviously the amplifier need notbe limited to three stages of amplification, the terminals 43 indicating means for attaching the filament of an additional stage to the potentiometer arrangement. More than one stage may be added in a similar manner.

i As hereinbefore stated, it is desirable in many cases to provide means for reinforcing particular frequencies such as the low frequencies which are normally suppressed by loud speaker characteristics and similar causes. Fig. 2 illustrates a three-stage directly coupled amplifier which is provided with such means. The power unit employed with this amplifier is entirely similar to that shown and described in Fig. 1 and requires no further explanation. Likewise as in Fig. 1,

the positive and negative leads 11, 12 from one side of the divided power circuit are connected across a potentiometer arangement from which the tubes may be operated. The amplifier proper comprises the three tubes 44, 45 and 46 which are directly coupled in cascade as in Fig. 1. The plate of tube 44 is connected through a resistance 47 to the point 48 on the potentiometer arrangement. The

plate of tube 45 is connected through a resistance 49 to the positive end 50 of the potentiometer arrangement. The plate of tube 46 is connected through the primary of the output transformer 51 to the positive lead 11'. The filament of tube 44 is shunted across a portion 52 of the potentiometer arrangement. The filament of tube 45 is operated from a transformer secondary 53, and its potential is controlled by means of a potentiometer 54 and lead 55 to the potentiometer arrangement. The filament of tube 46 is 0perated by a transformer secondary 56 and is connected bymeans of a potentiometer 57 to the negative lead 12 of the power circuit. The filament potential is controlled by means of a lead 58 from said negative lead 112'to a suitable point on the potentiometer arrangement, such as the point 48. The plate filament circuit of the tube 46 is preferably stabilized in any suitable way, as by means of the resistance 59. By-pass condensers 42 may be provided as described in connection with Fig. 1.

The means for reinforcing particular fre quencies preferably comprise an oscillating circuit of any suitable type. which is responsive to the desired frequencies, and on which a part of the incoming signalling energy is impressed. In said oscillating circuit the particular frequenciesconcerned are amplified, either by the resonant action of the circuit or preferably by the inclusion in said circuit of additional amplifying means, and said amplified frequencies are then returned to the input circuit of the amplifier proper, whereby the suppression of frequencies which is usually found in the output of the amplifier is obviated. Preferably, and in the form shown in Fig. 2, the oscillating circuit above described takes the form of an additional vacuum tube having its output circuit responsive at the desired frequencies and coupled back to the input circuit of the amplifier. Said vacuum tube may be and preferably is operated entirely from the potentiometer arangement, as in the case of the amplifying tubes of the amplifier proper.

Referring now particularly to the form shown by Way of example in Fig. 2, the input terminals and 61 of the amplifier, which normally supply the grid and filament of the first amplifying tube 44, are likewise connected to the grid 62 and filament 63 of an additional tube 64, preferably connected in parallel with the first amplifying tube 44 across the input circuit of the amplifier. The filament'63 is shunted across a suitable portion of the potentiometer arrangement and the plate 65 is connected through the primary 66 of a transformer 67 to a suitable point such as 68 on the potentiometer arrangement. A condenser 69 which is shunted across be tween the plate circuit of tube 64 and the potentiometer arrangement may be provided in order to obtain the proper selective frequencies of the plate circuit of said tube.

It will be observed that the input energy from terminals 60 and 61 is impressed simultaneously on the grids of tubes 64 and 44.

The plate circuit of tube 64 being adjusted to be resonant at, for example, thirty cycles,

will produce greatly amplified oscillations at and near said frequency, and said amplified oscillations will be sent back to the input circuit of the amplifier by means of the transformer 67. As a result thereof, the signalling energy impressed on the grid of the first amplifying tube 44 includes strongly reinforced low frequency components. Said signalling energy, after passing through three stages of amplification, passes through the output transformer 51 to a suitable loud speaker (not shown), whereby the relative over-emphasis which has been imparted to the low frequency components will be counteracted by the relative suppression normally encountered and the resulting sound waves produced will be practically free from distortion and of the same degree of amplification for practically all frequencies.

In case it is desirable to operate the amplifier without reinforcing the low frequencies, any suitable means may be provided for preventing the tube 64 from acting on the input circuit of tube 44. Preferably such means are variable in order to control the amount of reinforcement which is imparted to the low frequency components and may, for example, comprise a variable shunt resistance around one of the windings of the transformer 67. Referring to Fig. 2, the secondary of the transformer 67 is preferably by-passed by a suitable condenser 70, and

a suitable resistance 71 having a variable tap 72 is connected in shunt around said secondary. It will be apparent that as the resistance of said shunt path is increased, the reinforcing action of the tube 64 on the input to the tube 44 will likewise be increased, and that as the resistance is decreased the reinforcing action will likewise be decreased and may if desired be cut out altogether.

Jacks 73 inserted in the plate circuit of each tube provide a ready and convenient means for ascertaining the operating conditions of the various tubes and enable a quick adjustment of the amplifier to the desired operating point.

Fig. 3 shows the invention applied to a three-stage amplifier of the push-pull type. The power unit herein employed is similar to that shown and described in connection with Figs. 1 and 2, except that but one transformer is employed. The power is taken from the supply line, rectified, and impressed by means of the positive and negative loads 11, 12 on the potentiometer arrangement, all as more fully described in connection with Fig. l. The amplifier prop-er comprises the tubes 75, 76 and 77 and 75, 76, and 77', said tubes being connected in pairs in the well known push-pull relation. The input terminals 78, 78 are connected to the grids of the tubes 75, 75 respectively, and the input terminal 79 is connected through a resistance 80, preferably shunted by a condenser 80, to the filament heating circuit of said tubes, said resistance serving to maintain the grids negative with respect to the filament. The resistance 80 and the filaments of tubes 75 and 75' are preferably shunted by a condenser 80.

Filament heating current for the tubes is supplied by a transformer secondary 81, connected through any suitable type of rectifying unit 82, and choke S3 to the common heating circuit of the tubes. The plates of the tubes are connected directly to the grids of the respective succeeding tubes and also through resistances 84, 84 to a common lead 85 which is connected to the output terminal 86 and to the positive lead 11.

Heating current for the filaments of tubes 76, 76 is supplied by a transformer secondary 87, and the potential of the filaments may be adjusted relative tothe grids and plates by means of the potentiometer 88 and the lead 89 to the potentiomenter arrangement at a suitable point 90. The plates of tubes 76, 76 are connected directly to the grids of the respective succeeding tubes, and also through resistances 91, 91 with a common lead 92 which is connected with the output terminal 86 and to the positive lead 11.

Heating current for the filaments of tubes 77, 77 is preferably supplied by a transformer secondary 93, and the potential of said filaments relative to the grids and plates may be adjusted by means of a lead 94 connected to the potentiometer arrangement at a'suitable point 95. The plates of said tubesare connected respectively with the output terminals 96, 96. The operation of such circuits is well known in the art and will be readily understood without further explanation.

From the foregoing examples it will be readily apparent to those skilled in the art that the invention is not restricted to any particular type of receiving circuit, or to any particular number of amplifying stages, but is, capable of general application. It will be observed that in each case there is pro- Vided a potentiometer arrangement constituting a graduated source of D. C. potential which may serve as the sole source of power for a plurality of tubes constituting a multi stage amplifier, and is adapted to supply thereto the proper operating potential for all of the successive grids, plates and filaments of the amplifier. In combination with the directly coupled type of amplifier, there is provided an eflicient amplifier with a minimum of equipment and with maximum reliability and simplicity of voltage control and regulation. All blocking condensers, bias batteries, etc., are eliminated; the number of wires and connections is reduced to a minimum, thus eifecting economy in construction and reliability inoperation; the individual units of the system are largely limited to resistance units, which are cheap to construct, reliable, and readily obtainable on the market in all desired sizes and types; the system may be readily expanded to a larger number of stages or reduced to a smaller number with facility; etc. It will be apparent that all of the filaments may, if

desired, be operated directly from the potentiometer arrangement, instead of from transformer secondaries; that the invention is not restricted to any particular type of power circuit or supply; and that the divided power circuit shown in Figs. 1 and 2 may if desired be replaced by a single power circuit and potentiometer arrangement as shown in Fi 3.

ilhe amplifier thus produced constitutes a direct potential amplifier which possesses extreme accuracy, and has a substantially fiat characteristic and practically no distortion over a very wide band of frequencles.

At the same time the amplifier is free from all transformer distortion, trouble from blocking effects due to blocking condensers, etc., .and has practically notendency toward inductive or capacitive regenerating effect which might set up singing or howling in the tubes. 7

Furthermore, the provis'ionof means for reinforcing particular components, such as the extremely low' frequencies, insures an output'from'the sound reproducer or loud speaker which reproduces the original sound as exactly as possible. Such an amplifier gives exceptional results when connected to a radio receiving set, for example, or to any other source of electrical vibration produced by or corresponding to sound waves. The 4 amplifier is particularly well adapted, for

invention is not limited to the several em-' bodiments which have been described for purposes of exemplification only, and that changes may be made in the details of construction, the individual units, and their assembly and arrangement without departing from the spirit of the invention. Reference is therefore to be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. In I an amplifier, a potentiometer arrangement, a plurality of tubes in cascade each having its plate connected directly to the grid of the succeeding tube, the filament of the first tube being shunted across a portion of said potentiometer arrangement, the plate-grid circuit of the first and second tubes being connected to said potentiometer ar rangement at a point positive with respect to said portion, a resistance in said connection, filament heating means for said last-named. tube, a connection from said last-named filament to said potentiometer arrangement between said portion and said point of connection, a third tube having its grid connected directly in circuit with the plate of said second tube, a connection including a resistance between said last-named circuit and a point on said potentiometer arrangement, a connection from the filament of said third tube to said potentiometer arrangement at a point negative with respect to the point of connection of said last-named plate-grid circuit, and a separate potentiometer connected in circuit with the plate and filament of said last named tube. V v

2. In-an amplifier, a series of vacuum tubes in cascade, each of said tubes having its plate connected directly to the grid of the succeeding tubes, a potentiometer arrangement for operating certain of said tubes, connections from each plate-grid circuit and each filament .of saidlast-named tubes to said potentiometer arrangement, a separate source of powerconnected in circuit with the plate and filament of another tube of said series, and a connection between said last-named filament and said potentiometer arrangement.

3. In an amplifier, a series of vacuum tubes in cascade, each of said tubes having its plate connected directly to the grid of the succeeding tube,fa potentiometer arrangement for operating certain of said tubes, connections from each plate-grid circuit and each filament of said last-named tubes to said potentiometer arrangement, a separate potentiometer arrangement in circuit with the plate and filament of another tube of said series, and a connection between said lastnamed filament and said first-named potentiometer arrangement.

4. In a multistage audio frequency amplifier having means for impressing audio frequency energy thereon, means for reinforcing particular frequencies lying within the audible range comprising a separate amplifying circuit coupled to the input circuit of said amplifier and responsive at said frequencies within the audible range, said circuit being adapted to re-impress said amplified frequencies on said input circuit.

5. In an audio frequency amplifier, the combination of a plurality of stages in cascade and means for supplying audio frequency signalling energy thereto, with an oscillating circuit also supplied from said means and responsive at particular frequencies within the audible range, said circuit being coupled to the input of one of said stages.

6. In an amplifier, the combination of a plurality of stages in cascade, means for impressing audio frequency signalling energy on the input of said amplifier, and a separate amplifying circuit coupled to said input and adapted to amplify particular frequencies within the audible range and to re-impress the same on said input.

7. In an amplifier, a plurality of vacuum tubes in cascade, and an additional vacuum tube having its input in parallel with the input of one of said plurality of tubes and having its output circuit selectively responsive at a particular frequency or frequencies and coupled to the input of one of said plurality of tubes.

8. In an amplifier, a plurality of stages in cascade, means for impressing signalling energy on the input of said amplifier, a separate amplifying circuit coupled to said input and adapted to amplify particular frequencies, means coupling said circuit to the input of one of said stages, and means for shunting said coupling means.

9. In an amplifier, a plurality of vacuum tubes in cascade, an additional vacuum tube I having its input in parallel with the input of one of said plurality of tubes and having its output circuit selectively responsive at a particular frequency or frequencies, means coupling said output to the input of one of said plurality of tubes, and means for shunting said coupling means.

10. In an amplifier, a series of vacuum tubes in cascade, an additional tube having its grid in parallel with the grid of the first tube of'said series and having its output circuit selectively responsive at a particular frequency or frequencies, and means coupling said output with the input circuit of said grids. q

11. In an amplifier, a series of vacuum tubes in cascade, an additional tube having its grid in parallel with the grid of the first tube of said series and having its output circuit selectively responsive at a particular frequency or frequencies, means coupling said output with the input circuit of said grids, and means for shunting said coupling means.

12. In anamplifier, a series of vacuum tubes in cascade, an additional tube having its grid in parallel with the grid of the first tube of said series and having its output circuit selectively responsive at a particular frequency or frequencies, means coupling said output with the input circuit of said grids, and a variable resistance in parallel with said coupling means.

13. In an amplifier, a series of vacuum tubes in cascade, a tube external to said series and having its grid in parallel with the grid of the first tube of said series, a capacity and inductance in the output circuit of said last named tube, whereby said circuit is resonant at a particular frequency, and an inductance in the circuit of said grids which is inductively related to said first-named inductance.

14. In an amplifier, a series of vacuum tubes in cascade, a tube external to said series and having its grid in parallel with the grid of the first tube of said series, a capacity and inductance in the output circuit of said lastnamed tube, whereby said circuit is resonant at a particular frequency, an inductance in the circuit of said grids which is inductively related to said first-named inductance, and a variable resistance in parallel with said lastnamed inductance.

15. The method of amplifying audio frequency signals which consists in amplifying audio frequency signalling energy, separately amplifying particular frequency components of said energy within the audible range, and impressing said amplified components on the incoming audio frequency signalling eng 16. In an amplifier, a potentiometer arrangement, a series of tubes in cascade, means connecting the plates and filaments of said tubes to said potentiometer arrangement to provide operating potentials therefor, an additional tube having its plate and filament connected to said potentiometer arrangement and having its grid connected in parallel with the grid of a tube of said series, and

means coupling the output of said additional tube to the input of a tube of said series.

17. In an amplifier, a potentiometer arrangement, a series of vacuum tubes each having its plate connected directly to the grid of a succeeding tube, means connecting each plate-grid circuit and each filament with saidpotentiometer arrangement, an additional tube having its plate and filament connected to said potentiometer arrangement and its grid in parallel with the grid of one tube of said series, and means coupling the plate circuit of said additional. tube with the grid circuit of one tube of said series.

18. An amplifier comprising a plurality of tubes direct-connected in cascade, a potentiometer arrangement, means connecting the electrodes of said tubes to said potentiometer arrangement, an additional tube having its plate and filament connected to said potentiometer arrangement and its grid in parallel with the grid of one tube of said series, means coupling the plate circuit of said additional tube with said grids, and means for shunting said coupling means.

19. The combination with means including a series of cascade connected vacuum tubes for amplifying audio frequency signalling energy and means for supplying audio frequency energy to the input circuit thereof, of means connected with said second named means for receiving therefrom and amplifying particular components of said energy having frequencies within the audible range, and means for impressing-said amplified components on the input circuit of said firstnamed means.

20. In an alternating current amplifier, the

combination with a plurality of vacuum tubes direct coupled in cascade, of means for applying signal and operating potentials thereto, including an input circuit, a vacuum tube connected with said input circuit to receive signal impulses therefrom and having an output circuit connected with said input circuit to supply amplified signal impulses to the last named circuit, means in said connection responsive to a predetermined range of frequencies and for adjusting the amplitude of the signals applied to said input circuit therefrom, means for supplying rectified alternating current for heating the cathode of the first of said cascade connected tubes, separate means for supplying alternating current for heating the cathodes of each of the succeeding tubes, means connected between the cathode and the anode ofthe last tube of said series for supplying anode current thereto, separate means for supplying grid potential to the last named tube and grid and anode potentials to the preceding tubes, and means providing a connection between the cathode of the last tube of the series and a potential supply point on the last named potential supply means.

In testimony whereof I have signed this specification. l

HERMAN HOLLERITH, JR. 

